1. Field of the Invention
The present invention generally relates to a bar code symbol reading apparatus for reading bar code symbols by using a two-dimensional imaging device and, more particularly, to a data processing method for use in such an apparatus.
2. Description of the Related Art
Recently, bar code symbols are used in many countries. In Japan, for instance, bar code symbols are printed on about 99% of foodstuffs and daily necessaries, and the industries using bar code symbols have increased in number because of convenience, a high reading speed, and correctness of bar code symbols.
A number of types of bar code symbols themselves have been proposed. Representative examples of bar code symbols are a JAN/UPC/EAN code, an ITF (Interleaved 2 of 5) code, CODE39, an NW-7 code, and CODE128. In addition, two-dimensional bar code symbols having large information amounts and high recording densities are beginning to be proposed in recent years to meet demands for more information in a narrower space. Examples of the two-dimensional bar code symbols are CODE16K, CODE49, PDF417, and DATA CODE.
In contrast to these two-dimensional bar code symbols, the JAN/UPC/EAN code, the ITF code, the CODE39, the NW-7 code, and the CODE128 described above are often called one-dimensional bar code symbols.
Many types of apparatuses are available as an apparatus for reading the one-dimensional bar code symbols, such as the JAN/UPC/EAN code, the ITF code, the CODE39, the NW-7 code, and the CODE128. Representative types are a pen type, a touch type, and a laser type. However, methods of reading bar code symbols are almost the same. That is, light radiated from a light source onto a bar code symbol is reflected by the bar code symbol and focused by a light-receiving device incorporated in a reading apparatus. This focused signal is decoded by an electric circuit constituted by a microcomputer or the like.
The number of types of apparatuses, on the other hand, for reading the two-dimensional bar code symbols, such as the CODE16K, the CODE49, the PDF417, and the DATA CODE described above, is small. Therefore, a method of reading these two-dimensional bar code symbols has not been established yet. It is considered that the following two methods are currently general as a method of reading the two-dimensional bar code symbols. One is a method in which a bar code symbol is imaged by a two-dimensional imaging device and stored in an image memory (frame memory), and the contents of the stored bar code symbol image are decoded; the other is a method of decoding the contents of a bar code symbol by two-dimensionally scanning a semiconductor laser.
FIG. 6B shows the contents of an image memory (frame memory) after a general one-dimensional bar code symbol, such as the JAN/UPC/EAN code, the ITF code, the CODE39, the NW-7 code, or the CODE128, is imaged by a two-dimensional imaging device.
FIG. 6C shows the contents of an image memory (frame memory) after a two-dimensional bar code symbol, such as the CODE16K, the CODE49, the PDF417, or the DATA CODE, is imaged by a two-dimensional imaging device.
As is apparent from a comparison between FIGS. 6B and 6C, the shape of the two-dimensional bar code symbol is more complicated than that of the one-dimensional bar code symbol. In general, the recording density of the two-dimensional bar code symbol is 10 times or more that of the one-dimensional bar code symbol, and the recording capacity of the former is about 100 times that of the latter. The number of pixels of an image memory (frame memory) corresponding to a bar or space having a minimum width after a bar code symbol is imaged by a two-dimensional imaging device in the case of the two-dimensional bar code symbol is 1/10 times or less that in the case of the one-dimensional bar code symbol.
This big difference makes it very difficult to establish a two-dimensional bar code symbol reading technique compared to a one-dimensional bar code symbol reading technique. For example, whether or not two-dimensional bar code symbols can be read correctly depends on a slight error, such as an error in an angle at which a two-dimensional imaging device is mounted with respect to a two-dimensional bar code symbol (an error in a so-called read angle), an error in a height at which a two-dimensional imaging device is mounted with respect to a two-dimensional bar code symbol (an error in a so-called focusing state in reading), or an error in lighting conditions.
These errors, however, are not taken into account at all in conventional two-dimensional bar code symbol reading apparatuses.